For the generation of a proper T-lymphocyte (T-cell) immune response, two signals must be provided to the T-cell by antigen presenting cells (APC). First, antigen must be presented to the T-cell receptor (TCR) via a major histocompatibility complex (MHC), in an event that determines specificity. Second, an antigen-independent, costimulatory signal must be delivered by engagement of members of the B7 family on the APC with the CD28 protein on T-cells. A productive immune response leads to proliferation, differentiation, clonal expansion, and effect or function. In the absence of the second, costimulatory signal, T-cells undergo a state of long-lasting antigen-specific unresponsiveness, termed anergy.
T-cells initiate the immune response, mediate antigen-specific effector functions, and regulate the activity of other leukocytes by secreting cytokines. The T-cell receptor (TCR) distinguishes the T-cell from other lymphocytes and can bind antigen only when it is presented by the APC within the context of a MHC. The functional activity of a particular T-cell can be correlated with the expression of membrane antigens, such as CD4 and CD8. For instance, CD4+ T-cells generally function as T helper cells (Tc) and are MHC class II restricted, whereas CD8+ cells generally function as cytotoxic T-cells (Tc) and are MHC class I restricted.
Potent T-cell costimulatory polypeptides which has been previously identified include polypeptides termed B7.1 (Freeman et al. J. Immunology 143, 2714-2722 (1989), Freeman et al. Jour. Expt. Med. 174, 625-31 (1991)) and B7.2 (Freeman et al. Science 262, 909-911 (1993), and Freeman et al. Jour. Expt. Med. 178, 2185-2192 (1993)), (or CD80 and CD86, respectively). These polypeptides are either inducibly or constitutively expressed on various APCs and are membrane-bound ligands for CD28 and CTLA-4, respectively, on T-cells. CD28 (Aruffo and Seed Proc. Natl. Acad. Sci. 84, 8573-8577 (1987) and Gross et al. J. Immun. 144, 3201-3210 (1990)) is expressed on resting T-cells and mediates a positive costimulatory signal. CTLA-4 (Brunet et al. Nature 328, 267-270 (1987) and Dariavach et al. Eur. Jour. Immun. 18, 1901-1905 (1988)) expression is induced upon T-cell activation and negatively regulates the CD28 signal, due to its higher binding affinity for B7.1 and B7.2. Mice without the CTLA-4 gene exhibit dramatically high levels of T-cells, since the switch off mechanism for the proliferation signal is impaired in the absence of CTLA-4. This phenotype clearly demonstrates the major inhibitory effect that the CTLA-4 costimulatory protein has on T-cell proliferation. Mice lacking CD28 or B7.1 or B7.2 have a less severe phenotype, indicating that alternate pathways for T-cell costimulation may exist. There has been considerable interest in the CD28/CTLA-4 pathway as means for regulating T-cell activation and proliferation. A chimeric protein containing the extracellular portion of CTLA-4 fused to human Fc has strong immunosuppressive effects and has been studied in a variety of clinical settings. Antibodies to B7.1 and B7.2 proteins have also been evaluated for similar indications in the area of immunosuppression. Anti-CTLA-4 antibodies have shown utility in promoting T-cell activation. In addition, B7.1 and B7.2 gene therapy has shown great promise in the area of cancer immunotherapy.
Thus far, CD28, CTLA-4, B7.1 and B7.2 are involved in a single T-cell costimulatory pathway. Given the capability of modulating an immune response by regulating T-cell costimulation, it would be desirable to identify other members of the same or a separate T-cell costimulatory pathway which may have advantageous properties in regulating host T-cell function and immune response.
Accordingly, it is an object of the invention to provide novel polypeptides for stimulation of T-cell activity and/or proliferation. It is a further object of the invention to use the novel polypeptides for the prevention and treatment of T-cell mediated immune disorders.